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Sunday 15 April 2018

That famous "monolith extraction point" at Rhosyfelin

Perhaps the most frequently-cited of the "quarrying " or "engineering" features at Rhosyfelin is the so-called "bluestone monolith extraction point" near the tip of the spur, and very close to sampling point number 8 as examined by Rob Ixer and Richard Bevins.

Well, I was down in the valley again yesterday, and research continues.

Well there is not really a recess here at all, and given the nature of the bedrock above and below MPP's right hand, it is vanishingly unlikely that a bluestone monolith with dimensions 2m x 20 cm x 20 cm could have survived physical extraction by quarrymen from here, since the rock is criss-crossed with deep fractures. As I mentioned in a previous post, there are at least ten intersecting fracture planes within a very small area.  I'm amazed that the geologists did not point this out to Mike and his colleagues.  We are talking rock mechanics and common sense here.  

The other observation from my latest visit is that on the rock face from which the pillar is supposed to have been extracted, there is clear evidence that thin slabs of rhyolite have fractured and fallen away over a long period of time -- and not all on one occasion.  If you just look at the edges of the fractures on the rock face, some are quite young, others are moderately weathered and others are very old (rounded off and heavily abraded).  This little rock face has a complicated history, as any geologist or geomorphologist will confirm.  On the illustrations pasted below, click to enlarge.

And where is the debris from the flaky thin slabs or slices that have dropped away from this rock face?  Why, it has been thrown into buckets by the archaeologists and thrown away, on the basis that it was of no importance.  Very convenient.........

Much as I hate being a party pooper, there is only one conclusion to be drawn from these observations.  There is no monolith extraction point located here.   In other words, there has been no quarrying.

I'm convinced that if different parts of this face were subjected to cosmogenic dating analyses, the result would be a wide range of different exposure ages.  I believe that at least one sample was taken from here for analysis at Glasgow University a couple of years ago, and that the result should have been available last summer.  Whatever happened to that dating result?  Why has it not been published?  Why, in his 2017 lectures, did MPP not announce the date to the world?  

If the result had been favourable to the quarrying hypothesis, it would be out there by now, as sure as eggs is eggs.............. 

These "stumps" at the base of the rock face have been described as the solid rock remnants left behind when the monolith was levered away by the intrepid quarrymen.  But just look at how heavily abraded they are.  Like many other bedrock surfaces around the tip of the spur, they have been battered and worn down by powerful glacial meltwater streams loaded with sand, gravel, cobbles and even boulders.  The deposits left behind by these streams are located in close proximity to this point, and were exposed during the course of the archaeological excavations:


Myris of Alexandria said...

A few random comments.
Once again get the nomenclature correct you are showing joint planes. These are very different from fractures. It is disheartening to keep making this point.
It is not semantics but has geological implications.
I don't see any great difference in the degree of weathering but that were best argued in the field.
I suspect you are looking at the degree of limonite staining and equating that with weathering. It is not or rather only partly so.
Weathering is very superficial and ugh suficial (restricted to the surface)
However alteration WITHIN rocks is concentrated along and alongside joint planes.
This in turn is related to the local chemistry of the rock.

I agree that the extraction area of MPP et indeed very close to sample 8 of the pet rock boys.

BRIAN JOHN said...

According to all my text books joints are brittle fractures resulting from contraction or other stresses -- generally without any displacement, in which case they would be called faults. There might be lots of intersecting joints in a body of rock, but since each joint is a fracture we are simply splitting hairs. A fracture plane or fracture face is what we see when we look at a rock face and see a relatively flat face from which a mass of rock has fallen away or been dragged away.The plane may coincide with a foliation layer, or it may not -- you refer in your papers to "sub-planar fracture faces" -- by which I assume you mean that the faces left behind do not quite coincide with the foliation layers. I don't see any reason at all to change any of my terminology.

Weathering -- no, I am not just looking at staining (which we might refer to as chemical weathering) but at the physical degradation of the rock itself -- and we would call that physical weathering. If you enlarge those pics and take a careful look, you will see what I mean.

TonyH said...

MPP still reminds me of the former TV academic, David Bellamy, in this pose and photo!

BRIAN JOHN said...

..... whom we of Durham remember all too well!!